Large format 2D and 3D cinematic projection has been providing audiences with an immersive theatre experience since the 1970's, and the projection technology is well established. The large format (70 mm) commercial exhibitor benefits from the capability to present both two-dimensional (“2D”) and three-dimensional (“3D”) cinematic presentations from the same projection system. This increases his/her return on investment. The operator would clearly benefit if the projection system functioned efficiently in both the 2D and 3D operating modes.
There can be technological differences between standard 355 mm and large format 70 mm projection equipment. The large film frame dimension offered by the 70 mm/15 perf format drives all these differences. The size of the large format film frame is about ten times that of the standard 35 mm film frame. Because of this, almost everything about the large format projection systems is generally larger, faster, or more powerful than their standard 35 mm cousins. A defining feature of the large format technology is the powerful illumination system required to illuminate the horizontally traveling 70 mm wide film.
The illumination system in a projection system represents a significant factor in the cost of operating the system. The arc lamps have limited lifetimes (1000 hours), and so must be continually monitored and periodically replaced when they no longer meet performance requirements. Lamp replacement is a potentially hazardous task that requires a careful alignment procedure conducted by a trained individual.
The high power lamps consume significant amounts of electrical power and generate tremendous amounts of heat. This heat is generally vented from the projection room and out of the building, and air conditioning must be adequate to cool the small projection hall. This generates increased utility costs for the exhibitor. Shutting down lamps between shows to conserve utility costs is often not a viable option. Standard high power arc lamps generally cannot be extinguished and restarted without significant penalty to the arc lamps lifetime (1.5 to 2.5 hours per lamp start).
For 3D projection, the demands on the illumination system may be more than doubled. In some cases two channels, one for each eye, are projected simultaneously from two physically separate but synchronized projectors, each with its own film reel. Each channel may be polarized with a different polarization, and the two polarizations are orthogonal to each other. In other cases, a single channel is used to project each eye in sequence. With a single channel 3D projector, the projector may have a polarizer that is capable of changing for each eye or active LCD glasses are used that are synchronized to the images being projected. The polarization of the illumination results in a light loss of over 50% as compared to the non-polarized 2D projection, while the screen brightness requirements remain unchanged. Using active glasses also results in a light loss compared to 2D projection. This results in a significant difference in screen luminance between 3D and 2D presentations. Standard arc lamps can only be operated near their full output power (to conserve lamp life), so modulating the lamp power to compensate for the varying 2D and 3D power requirements has not been a viable option with existing systems.
For long duration 2D projection, there are additional light inefficiencies. Limits to the physical size of reel units that hold the film demand that these long duration presentations be split between two distinct reel units. The first part of the presentation is played back through one channel of the system with a transition to the second channel for the final part of the presentation. The penalty associated with lamp ignition normally leads to the requirement that both upper and lower channel lamps remain on during the whole presentation.
The large film format may demand not only a significantly more powerful illumination system, but also one that delivers the required uniformity and stability over the much larger film frame of the 70 mm format. The performance requirements of the large format illumination system exceed that of the standard 35 mm systems.
Some conventional projection systems have utilized multiple lamps. For example, U.S. Pat. No. 4,916,485 discloses a projection system with side-by-side lamp houses that can be used for both 3D and 2D projection of large format cinema. While this projector system uses two lamps, there is only one lamp for each channel, which offers no advantage over standard stereographic projection systems. Particularly, there is no way to balance the light levels between 2D and 3D operating modes without incurring a significant loss in efficiency.
U.S. Pat. No. 3,914,645 discloses a multiple lamp unit for use with a photographic projector. The '645 patent provides for a single lamp projector with multiple “back up” lamps mounted on a turntable that can be rotated so as to move successive lamps into working position to automatically replace lamps when they fail. In U.S. Patent Application No. 2003/0128427 a system for employing dual projector lamps is disclosed. It uses two sources and polarization optics to select between one source and the other, using one source at a time. U.S. Pat. No. 6,545,814 discloses a method for combining multiple arc lamp sources for a electronic projector using prismatic structures integrated onto an integrating rod.
U.S. Patent Application No. 2002/0145708 discloses a dual lamp projector illumination system with a broad spectrum source and a narrow spectrum source. The narrow spectrum light source is an LED and is used to complement the spectrum of the broad spectrum source, which has a spectral power deficiency. U.S. Pat. No. 5,997,150 discloses a multiple emitter illumination engine with a holographic diffuser with particular application to xerographic printers and for illuminating spatial light modulators with high intensity light. In U.S. Pat. No. 6,341,876, a method for combining two lamps into a light pipe is disclosed. The '876 patent discloses the use of a parabolic reflector for the arc lamps. The '876 patent also discloses a method of combining the output of two lamp sources into a light pipe with two right angle prisms.
U.S. Pat. No. 5,504,544 discloses a method for combining multiple lamps using a series of Fresnel collecting and focusing elements. U.S. Pat. No. 4,372,656 discloses a single lamp projector that can be used for 3D as well as 2D projection through the introduction of a polarization device.
These prior projection systems do not disclose the balancing of light levels between the 3D and 2D operating modes of a projection system nor do they address the optimization of efficiency and reduced operating costs for 2D and 3D operation of these projection systems.
Temporal stability of the light output in the frequency range over which the human visual system is sensitive is an important projection system attribute. Flicker and shimmer are the product of the frequency dependent sensitivity of the human eye times the stability of the light output. Flicker is the global fluctuations of light levels at the screen. Flicker is present when the total luminous flux output from the projector varies with time. Shimmer is localized spatial fluctuations at the screen. When shimmer is observed, the illuminance changes locally on the screen despite a constant total luminous flux output from the projector. Thus a decrease of illuminance in one area on the screen is compensated for by an increase in illuminance elsewhere on the screen.
Arc lamp induced temporal instabilities present a particular challenge to the illumination system of an arc lamp based projection system. These instabilities can manifest themselves as flicker and shimmer of the projected image. Human perception is particularly sensitive to these fluctuations, and people are able to discern temporal fluctuations as small as one part in two hundred. This places a far more stringent requirement on the illumination system than does the requirement for static illumination uniformity across the screen. Shimmer and flicker are kept below the human detection threshold in order not to detract from the presentation.
Arc lamp instabilities can be caused by modulation of the arc's position and shape within the lamp envelope of the lamp. These modulations induce spatial and angular variations of the illumination signal. Turbulence within the envelope induces other localized angular deviations as the illumination signal propagates through the turbulent regions. These temporal angular modulations of the illumination at the lamp are transformed to angular and spatial fluctuations of the irradiance patterns in subsequent positions of the optical system, which in turn are perceived as shimmer or flicker by the audience.
The level of temporal instability of an arc lamp becomes more acute as the power of the lamp increases and its size decreases. Arc lamp stability is also known to degrade with lamp age. To meet the illumination requirements of large screens, high power lamps are employed. To satisfy- the demands of a compact projection system, there is a drive to make the lamps as small as possible. The higher levels of convection within the envelope of a compact high power lamp lead to a greater amount of temporal instability.
Arc lamp output fluctuation is a recognized problem, and there are several examples of conventional solutions relating to its reduction. These solutions generally involve modifying or manipulating the electrical power characteristics driving the lamp, for example, U.S. Pat. Nos. 6,525,491, 6,479,946, and 6,239,556, or modifying the ingredients within the lamp envelope, for example, Japanese Patent Application No. 02-01-01 01035447, and Japanese Patent Application No. 00-77-76 05151932.
Optical means to reduce shimmer is also used by some conventional solutions. Japanese Patent Application No. 03-01-00 00066135 discloses that a number of discrete “half mirrors” to flatten the light fluctuations caused by the shimmer. In Japanese Patent Application No. 00-95-76 56149180 a photochromic device is applied with a feedback circuit to control the transmission of the photochromic device.
U.S. Pat. No. 6,341,876 discloses a method for optically eliminating the effects of shimmer from the projected images. The '876 patent discloses a condensing lens at the input of a light pipe with the express intent of eliminating the image of the turbulent region within the arc lamp at the output of the light pipe.
In the paper entitled “Design Improvements for Motion Picture Film Projectors,” C. L. DuMont et al., SMPTE Journal, vol. 110, no. 11, 2001, the authors present results of their work in applying fly's eye integrators to 35 mm cinematic projectors. The paper discusses the advantages that the fly's eye integrator provides in reducing the lamp-induced shimmer in the projected image. They also discuss the use of a Cermax sealed beam lamp in the projection system.
U.S. Patent Application No. 2003/0142296 discloses a means for monitoring light levels by using a detector plus integrating box plus mirror assembly located behind a primary mirror that reflects a large portion of the visible light towards a light imaging device. This application discloses that it is necessary to sample and integrate 10% to 50% of the light transmitted by the primary mirror in order to achieve a sufficient signal to noise ratio.
U.S. Pat. No. 5,818,575 discloses a method to detect instability in an arc lamp's spatial distribution, particularly for use in lithography projection optics. At least two detectors are placed laterally across the illumination field at the wafer plane or conjugate to the wafer plane. The ratio of the output from the two detectors indicates the stability of the arc lamp.
These references do not disclose a light efficient and cost effective means of suppressing lamp-induced shimmer and flicker in the projected image. As described above, these modulations may be at a higher magnitude than usual due to the use of compact high wattage lamps. While fly's eye and light pipe homogenizers reduce these fluctuations, limitations in the fabrication methods as well as efficiency considerations make sufficient homogenization impractical and inefficient.
Additionally, the large physical size of the typical 70 mm format projection system can make them incompatible with standard 35 mm projection facilities. The vast majority of theatre venues are designed for the standard 35 mm format projection systems. Theatre operators considering the installation of modern large format projection equipment must therefore factor in renovations to convert existing 35 mm projection halls. This may increase the installation costs, disrupt theatre operations, and prolong the installation process. These factors may all contribute to increased cost of ownership to the theatre operator.